Fired, But Not Forgotten

A misconception about the AIM-120 is that the missile is a pure fire-and-forget missile, much like the AIM-9 Sidewinder. Although the AMRAAM's active radar-homing seeker does feature this capability, its ability to guide itself in this manner is limited by the distance between itself and the target. Simply put, the small radar located in the missile is not powerful enough to detect and track a target over the ranges at which missile launch typically occurs. At these ranges, the missile requires periodic updates during the flight as to the target's range and position.

Though not completely autonomous in particular circumstances, the AIM-120 is a vast improvement over its predecessor, the AIM-7 Sparrow. This semi-active radar-homing (SARH) missile required the launch-aircraft's radar to continuously illuminate the target. The Sparrow then guided itself by following the radar energy reflected by the target aircraft. Should the launch-aircraft fail to keep the aircraft locked on radar, the missile would go astray. Using the AIM-120, the launch-aircraft is free to maneuver or direct attacks against other targets. In addition, the AMRAAM also allows the pilot to direct simultaneous attacks against multiple targets.

A typical engagement starts with the aircraft's radar detecting and tracking the target. Just prior to launch, the launch aircraft's fire-control software will download the inertial data for the target to the missile. This provides the AIM-120 with navigation data allowing the AMRAAM's onboard autopilot to guide the missile to the predicted location of the target. Updates from the launch aircraft provide mid-course corrections to the missile in order to provide the most accurate range and bearing data to the autopilot.

Once the missile has reached the destination indicated by the autopilot, the AIM-120's onboard radar engages in an active search for the target. Once acquired, the radar will guide the missile the final distance to the target. Because the radar only activates during the terminal phase of the missile's flight, the target receives little warning. As a result, the opposing pilot has only a few seconds to engage countermeasures and/or begin evasive maneuvers.

In addition to this mode, the AMRAAM is fully capable of directing itself to a point in space from which it will engage its radar to seek out the target. However, without periodic updates there is an increased risk that the missile will be unable to acquire the target because the aircraft will have moved out of the seeker's scan envelope. Despite the disadvantages, this mode does allow the aircraft to launch an AMRAAM and then disengage from the battle entirely.

Guidance System

The Weapons Guidance Unit (WGU) of the AIM-120 consists of a high powered transmitter-receiver, a low-sidelobe antenna and accompanying servo, an Inertial Reference Unit to aid in autonomous navigation, and frequency processor. The entire package is contained within a sealed environment to protect the delicate equipment from the elements. Signals from the guidance package are transmitted to the steering fins via the fairing running down the airframe. Mounted inside the tail of this fairing is the datalink antenna. Mid-course corrections from the aircraft are fed to the missile's Inertial Reference Unit via this small antenna.

The AMRAAM's radar system operates in the 8 to 10 GHz I-band region. Its frequency processor provides the AIM-120 with two detection modes; high PRFs for locating distant targets and medium PRFs, giving the missile a look-down / shoot-down capability against low-flying targets which might otherwise be obscured by the terrain below. Should the target attempt to protect itself with active jamming, the radar is capable of utilizing its medium-PRF mode to provide a "home-on-jam" function.

Fusing System

While capable of detonating on impact, it is rare for a missile to actually come into direct contact with a maneuvering aircraft. As a result, modern air-intercept missiles are equipped with a proximity fuze. Designed to detect an object within the blast-radius of its warhead, the system detonates the warhead and relies on the blast and shrapnel effects of the missile to eliminate the target.

The active-radar proximity fuze of the AIM-120 is located just forward of the 40-pound warhead. It has been reported that the proximity detector is capable of determining where the target is in relation to the missile and then directing the blast and fragmentation pattern in that direction, thus maximizing effectiveness. However, this feature has not been confirmed, nor has the lethality range of the AIM-120's warhead.

Processing Power

Controlling the entire guidance package, as well as the proximity fuze, navigation, autopilot, and all self-testing features is a single 30MHz microprocessor. This electronic heart of the missile is located just forward of the fuzing system. A series of batteries provide power to the electronics and are located immediately aft of the antenna, surrounded by the transmitter system._